As disclosed in Japanese Patent Kokai Nos. 135,992/91 and 139,288/91, AA-2G is a type of saccharide derivative of L-ascorbic acid with a chemical structure represented by Chemical formula 1, which has a satisfactorily high stability but no reducing power. Further, AA-2G is easily hydrolyzed in living bodies to exhibit biological activities inherent to L-ascorbic acid.

Japanese Patent Kokai No. 183,492/91 discloses a process for producing AA-2G, which maybe carried out in an industrial scale. The process comprises the steps of:
allowing a saccharide-transferring enzyme with or without glucoamylase (EC 3.2.1.3) to act on a solution containing as substrates L-ascorbic acid and an α-glucosyl saccharide to form AA-2G and by-products;
subjecting the resulting reaction mixture containing AA-2G together with other by-products and remaining substrates to a column chromatography using a strongly-acidic cation exchange resin to collect from the eluate one or more fractions which are rich in a high AA-2G;
concentrating the obtained fraction(s) into a supersaturated solution; and
crystallizing and collecting AA-2G in the solution.
However, it is known that in the case of using cyclomaltodextrin glucanotransferase (EC 2.4.1.19) (abbreviated as “CGTase”, hereinafter) as the saccharide-transferring enzyme, 5-O-α-glucopyranosyl-L-ascorbic acid as represented by Chemical formula 2 (abbreviated as “AA-5G”, hereinafter,) and 6-O-α-glucopyranosyl-L-ascorbic acid as represented by Chemical formula 3 (abbreviated as “AA-6G”, hereinafter), both of which are structural isomers of AA-2G, are inevitably formed as by-products together with AA-2G. Also, it is known that in the case of using α-glucosidase as the saccharide-transferring enzyme, AA-6G is formed as a by-product together with AA-2G. L-Ascorbic acid and glucose, which are present with AA-2G in reaction mixtures, can be easily separated from AA-2G by a column chromatography using a strongly-acidic cation exchange resin because AA-2G is distinct from L-ascorbic acid and glucose with respect to with their molecular weight. However, AA-5G and AA-6G, formed as by-products along with AA-2G, are hardly separated from AA-2G because they have the same molecular weight. Thus, the presence of AA-5G and AA-6G may prevents the purification of AA-2G, as well as inhibit the subsequent crystallization of AA-2G in supersaturated solutions.

Furthermore, when crystalline AA-2G is collected from the first massecuite and the remaining mother liquor is then subjected to the second and third crystallization steps, AA-5G and AA-6G (isomers of AA-2G) in the mother liquor may inhibit the crystallization of AA-2G to reduce its yield of the second and the third crystallization steps.
As disclosed in Japanese Patent Kokai No. 117,290/93, AA-5G and AA-6G can be eliminated by advantageously utilizing their oxidizabilities which originate from their reducing powers. Particularly, a high AA-2G content product can be produced through the steps of oxidizing such an isomer with a reducing power where reaction mixtures containing the isomer and AA-2G are subjected to an effective oxidizing treatment; and separating AA-2G from the mixtures which contain AA-2G together with the resulting oxidized derivatives of AA-5G and AA-6G.
However, in order to subject a reaction mixture containing AA-2G and isomers with a reducing power to an oxidizing treatment and to attain a prescribed oxidization, it is necessary to chose conditions for oxidization which do oxidize the isomers while leaving AA-2G intact; For example, one can employ a method where the reaction mixture is exposed to aerobic conditions by aeration and agitation. Such a treatment inevitably requires complicated handling and controls; for example, consistently keeping the reaction mixture at a slightly acidic or alkaline pH level; admixing an oxidation-promoting agent such as metal salts including copper salts, iron salts, and the like, active charcoals such as steamed charcoal and zinc chloride charcoal with the reaction mixture; or admixing an oxidizing agent such as hydrogen peroxide, potassium permanganate, etc. with the reaction mixture. If oxidizing treatment is insufficient, isomers with reducing power still remain in the reaction mixture. On the contrary, if oxidation is excessive, AA-2G is affected, resulting in a reduced yield for AA-2G rich products. Oxidation treatment is usually required him or her to accurate control of the reaction condition to keep progress of oxidization at a prescribed level. Because of these, the above described treatments become very costly.
The present invention is established to solve the disadvantage of the conventional process for producing AA-2G described above. The present invention provides a process for producing AA-2G, not requiring the separation of the structural isomers and enabling the efficient production of AA-2G.